Three basic types of steel conduits are in use today: Rigid Steel Conduit, Intermediate Metal Conduit and Electrical Metallic Tubing.
Rigid Steel Conduit (RSC), also known as Rigid Galvanized Steel (RGS) or Galvanized Rigid Conduit (GRC), is a thick-walled threaded tubing, with a tubing wall that is thick enough to allow it to be threaded. Its common applications are in commercial and industrial construction.
Intermediate Metal Conduit (IMC) is a steel tubing heavier than EMT but lighter than RSC. It may be threaded.
Electrical Metallic Tubing (EMT), sometimes called thin-wall, is commonly used instead of RSC, as it is less costly and lighter than RSC. EMT itself is not threaded, but can be used with threaded fittings that clamp to it. Lengths of conduit are connected to each other and to equipment with clamp-type fittings. Like RSC, EMT is more common in commercial and industrial buildings than in residential applications. EMT is generally made of coated steel, though it may be aluminum.